Patella Instability 

Stability is afforded by a complex interaction between soft tissues and bony structures.

Anatomically these can be divided into 3 groups

-       Active stabilisers 

                  o  Components of quadriceps femoris 

                  o  Affected by loss of muscle control due to weakness leading to patella tilt

-       Passive stabilisers

                  o  Medial retinaclulum as the medial patellafemoral ligament

                  o  Laxity or rupture of MPFL compromises stability

-       Static Stabilisers ie. Bony anatomy

                  o  Influenced by trochlea dysplasia, patella alta, lateralisation of patella tendon insertion

Natural History of Patellar Instability

The natural history of untreated patellar dislocation demonstrates that in patients with first-time patellar dislocations treated conservatively, the chance of recurrent dislocation ranges from approximately 30% to 40%.  The redislocation rate is associated with younger age with a rate of up to 60% in patients younger than 15 years compared to 33% in patients aged 15 to 18 years.  First time traumatic patellar dislocation has an overall incidence of osteochondral fractures of 24%.

Treatment of Patella Instability

The recommendation is that first-time traumatic patella dislocations be treated initially with nonoperative measures unless there is clinical, radiographic, CT, and/or MRI findings of chondral injury, osteochondral fractures, or large medial patellar stabilizer defects (MPFL, medial retinaculum, VMO). Arthroscopy should be performed if chondral injury or osteochondral fracture is suspected. If the osteochondral fracture is greater than 10% of the patella articular surface or part of the weight-bearing portion of the lateral femoral condyle, open repair should be performed as long as the fragment is amendable to fixation.  Large soft tissue medial patellar stabilizer defects should  undergo open repair or reconstruction, especially in patients with a high level of athletic participation. All patients with first-time traumatic dislocations should be suspected as having an osteochondral injury until proven otherwise by MRI, CT scan and/or continued clinical examinations of both the injured and contralateral knee.

Physiotherapy is the first line treatment in patients with uncomplicated patella instability.  Strengthening of the quadriceps muscle, and in particular, the VMO component is the key to improving patella stability.  

Indications for surgery are osteochondral fractures, and recurrent instability in spite of a good physio programme.

There are no long-term studies in the English language with an adequate number of patients reporting results of acute surgical repair of the MPFL in first-time patellar dislocations. It is reasonable and becoming more accepted to think large defects or avulsions are not going to heal or  have a good functional outcome with closed treatment especially in individuals with high-level athletic participation and those with evidence of one or more predisposing factors.